Specs cameras are painted yellow and are fitted into normal camera housings with what looks like a light on the side. As GTB mark says, there have to be a pair of them across a set distance to work out your average speed.
The cameras you have noticed are probably ANPR Units ...(Automatic Number Plate Recognition)..
Basically they scan and log your registration against the national police computer to see if your car is reported as stolen..
Most larger town centres have them now, but you dont always notice them..(unless your in the trade)
hope this helps
jon